1. Field of the Invention
The present invention relates to improvements in the intermittent type swirl injection nozzle for atomizing fuel using a tangential passage and a swirl chamber to supply a nearly conical fuel spray through a nozzle hole, and more specifically to an intermittent type swirl injection nozzle which is capable of regulating fuel spray angle, spray penetration (the reach of the spray) and atomization characteristics corresponding to the lift of the needle valve and thus most suitable for use as the swirl injection nozzle designed to improve the performance of a direct-injection internal combustion engine of a single-hole, bowl-in type particularly in terms of reduction in smoke concentration.
2. Description of the Prior Art
The recent tendency is to provide more compact smaller cylinder capacity), higher-speed internal combustion engines for motor vehicles and consequently there has been demand on the introduction of fuel injection systems of renovated performance. In other words, the fuel injection system is required to be always adaptable to each operating condition in order to maintain the optimum performance of the engine in a wide range of operating conditions with regard to the engine speed and the load.
For example, fuel injection systems for diesel engines mainly comprise injection pumps, injection pipes and fuel injection nozzles. It is well known that the spary characteristics of the fuel injection nozzle among them directly affect the performance of the engine.
In the case of a conventional direct injection type internal combustion engine, the fuel injection nozzle is arranged nearly in the center of a recess formed in the top face of the piston to allow fuel to be sprayed radially from a plurality of nozzle holes. The intake swirl generated by the intake valve and the intake passage when air is taken in the engine still exists even in the last stage of compression stroke and forms a fuel-air mixture while pushing the fuel spray in the swirling direction within the recess. The diameter of the recess employed is usually set at 40 to 70% of that of the piston or cylinder. Accordingly, the diameter of the recess, in the case of a small engine whose piston diameter is less than 100 mm, is small and will be made even smaller if the compression ratio has to be greater. As a result, the fuel radially injected out of the plurality of nozzle holes of the fuel injection nozzle collides with the inside wall of the recess and adheres to the wall to form liquid films on the wall surface or remains as coarse large droplets, causing the decrease of effectively combustible air-fuel mixture. Accordingly, the disadvantages include not only decrease in output and fuel-efficiency but also the emitting of smoke.
Although the application of a swirl injection nozzle as one of the fuel injection nozzles to direct-injection engines has been attempted by the present inventors and proved utilizable, it has also been recognized necessary to provide the different spray characteristics of the swirl injection nozzle corresponding to engine operating conditions for obtaining optimum engine performance by forming an ideal fuel-air mixture over a wide range of operation. Accordingly, that recognition has made it essential to develop a swirl injection nozzle capable of providing different spray characteristics depending on engine operation conditions.
In the case of an intermittent type swirl injection nozzle, there are methods of imparting swirling movement of fuel by (A) providing a needle valve 1 with a swirl passage 4 in the periphery thereof as shown in FIG. 1; (B) a tangential port 6 tangentially communicating with a swirl chamber 5 as shown in FIG. 2; and (C) a swirl passage 10 in the periphery of the partition member 9 provided between the inner wall 8 of a nozzle body 7 and the needle valve 1 as shown in FIG. 3. The following description will be made as to the nozzles (A) and (C) as the prior art. Since the nozzle (B) is basically similar to (A) or (C), the description of (B) will be omitted. In the intermittent type swirl injection nozzles A and C, the fuel is swirled in the swirl passages 4, 10 and jetted out of the spray hole before being atomized to form spray. Since the injection nozzles A and C as compared with the standard injection nozzles of hole type and throttle type offer a large spray angle and excellent atomizing characteristics, the reach of the spray is short. In the conventional intermittent type swirl injection nozzles A and C, the spray angle .alpha. is enlarged as the lead angle .theta. formed by the axial lines of the swirl passages 4, 10 and the needle valve 1 is made larger.
However, because the fuel flow resistance of the swirl passages 4, 10 increases in the conventional intermittent type swirl injection nozzles A and C, the sufficient quantity of spray is unavailable when the internal combustion engine is operated at high speed.
On the other hand, if the injection pressure is raised, the velocity of the fuel flowing through the swirl passages 4, 10 of the conventional intermittent type swirl injection nozzles A and C and the inlet swirl velocity at the spray hole 14 will be increased. Consequently, the spray angle .alpha. is larger, which will not allow the penetration to increase. To obtain a satisfactory fuel-air mixture formation in the single-hole bowl-in type direct-injection internal combustion engine wherein fuel is tangentially diagonally injected into the roughly 3/4 spherical combustion chamber, the penetration of the spray and the divergence angle .alpha. must respectively be decreased and increased (40.about.60.degree. ) while the engine is operated at low speed, whereas the penetration must be greater than what is available at the time of low speed operation, with the smaller angle .alpha. while the engine is operated at high speed. For these requirements, the fuel injection pump (of line plunger type and distributor type) shows such characteristic that the injection pressure becomes extremely large with the higher engine speed even at the same injection quantity. Accordingly, in a fuel injection system comprising a combination of the intermittent type swirl injection nozzle and fuel injection pump, the lead angle .theta. (6.about.10.degree.) has had to be determined, in consideration of the nozzle characteristics, so as to let the swirl injection nozzle offer a smaller angle .alpha.(30.about.40.degree.) at the lower engine speed in order to avoid the conditions that the angle .alpha. is excessively large and the penetration is too small at the higher engine speed. In the conventional intermittent type swirl injection nozzles A, C, shown in FIGS. 1 and 3, if the gap t.sub.o between a needle valve 1 and a nozzle hole 2 is set small (normally 4/100-2/100 mm) and if the lead angle .theta. (normally 20.about.60.degree.) is set large, the spray angle .alpha. will become as large as 50.about.60.degree., allowing the performance of an engine to be improved while it is operated at low speed because the atomization of fuel is bettered and the fuel is prevented from colliding with the wall of the combustion chamber and the density of smoke discharged from the engine is minimized. On the contrary, the flow resistance of the swirl passages 4, 10 increases while the engine is operated at high speed, causing the sufficient quantity of injection to be unavailable, the angle .alpha. of divergence to be excessive and the penetration of the spray to be suppressed. As a result, the performance of the engine will be reduced.
On the other hand, if the gap t.sub.o is increased (5/100.about.8/100 mm), the fuel quantity not allowed to pass through the swirl passages 4, 10 will increase, causing the swirling velocity at the inlet of a spray hole 14 to be reduced and the spray angle .alpha. to remain as it is, whereby reduction in the performance of the engine while it is operated at high speed can be prevented except for its operation at low speed. As for the fuel flowing through the gap t.sub.o, it is negligible at low engine speed where the injection pressure (fuel pressure) is low because the thickness of the boundary layer is thick but the fuel flow rate in this portion is seen to increase at high engine speed where the injection pressure is increased because the thickness of boundary layer is decreased. Accordingly, to enlarge the spray angle .alpha. at low speed operation and decrease the angle .alpha. at high speed operation or to obtain a sufficient spray angle .alpha. when the injection quantity or needle valve lift is small, it can be accomplished by setting the gap t.sub.o changeable with the lift of the needle valve 1 (with the same injection volume, the lift of the needle valve decreases at the low speed operation, allowing the needle valve lift to reach its maximum value at the medium speed operation, whereby the needle valve lift can be maintained constant even if the engine is operated at higher speed). Thus the gap t.sub.o provided between the needle valve 1 and the valve hole 2 affects the spray angle .alpha. and further various kinds of performance of the swirl injection nozzle.
As matter of fact, the present inventors made a series of experiments and analyses in consideration of the aforementioned problems to improve the gap t.sub.o, the spray angle, the spray hole and the like and obtain spray characteristics corresponding to internal combustion engine operating conditions.
Although consideration has generally been given to the aforementioned gap t.sub.o only to prevent fuel oil leakage, the present inventors deem such a gap as an essential condition affecting the performance of a swirl injection nozzle. In other words, they have devised an intermittent type swirl injection nozzle for implementing functions heretofore not in existence by establishing a novel method for utilizing the gap as a fuel passage to make the swirl injection nozzle demonstrate its optimum performance in terms of the spray angle, flow rate coefficient and the reach of the spray corresponding to the internal combustion engine operating conditions.